Medical devices have been provided for treating tissue of a mammalian body by the use of radio frequency energy. See, for example, U.S. Pat. Nos. 5,370,675, 5,385,544 and 5,549,644. Radio frequency energy passing from an electrode of such a device through the adjoining tissue causes heating of the tissue. In a monopolar device, the radio frequency energy passes from the active electrode to an indifferent electrode typically in contact with the exterior of the body of the patient. In order to cause desired tissue ablation and subsequent necrosis, the treated tissue is heated to a temperature in excess of approximately 47° C. However, if the temperature of the tissue being treated is elevated too high, dehydration and later charring of the tissue can occur. Such dehydration and charring can increase the impedance of the tissue to a level that prohibits radio frequency from traveling through the tissue. In view of the foregoing, prior systems have monitored the impedance of the active electrode circuit and adjusted the radio frequency power supplied to the electrode in response to such impedance measurements.
It would be desirable to provide a method and apparatus that is able to predict whether the impedance of an active electrode circuit will exceed a predetermined level during a procedure and adjust the power supplied to such circuit so that such predetermined level of impedance is not reached during the procedure.